The present invention relates to an impact dot printer, and more specifically, relates to a circuit for driving a head of an impact dot printer and to a power control technique for controlling a power source for a head drive circuit.
To perform printing, an impact dot printer drives a print wire by using, for example, the magnetic attractive force of an electromagnet. FIG. 8 is a diagram showing an example wire impact print head for the print head of the thus arranged impact dot printer.
In the example in FIG. 8, a wire impact print head 51 has a plurality of wires 57 that are attached, by wire levers 53 and return springs 55, so that they reciprocate. When a drive current flows through a head coil 59, a wire lever 53 is attracted by the magnetic attractive force produced by the electromagnet in the direction indicated by an arrow in FIG. 8, and a wire 57 strikes an ink ribbon 61 and forms dots on a printing sheet 65 moved in consonance with the rotation of a platen 63.
FIG. 9 is a diagram illustrating the fundamental structure of the circuit of the print head 51 for driving the head coil 59. In this example, only one head coil 59 and head drive transistor 33 set is shown, but in actuality, a plurality of these sets are provided. A drive circuit (driver) 30 for each head coil 59 is constituted by one of the head drive transistors 33, a head drive power source 34 and a Zener diode 35. During a predetermined conductive period, a control pulse 32 is maintained at level H by a print controller 31, and a pertinent head drive transistor 33 is maintained in the ON state (in the saturated region). Then, a voltage (e.g., 35V) supplied by the head drive power source 34 is applied to the head coil 59, and a drive current i1 flows through it. Thereafter, when the control pulse 32 falls to level L, the head coil 59 generates an inductive electromotive force to render off the head drive transistor 33. For this, the Zener diode 35 is rendered conductive at the induced voltage, and a base current flows to the head drive transistor 33, while the head drive transistor 33 enters a linear operating region. Subsequently, the drive current i1 flows through the head drive transistor 33 and the current value is drastically reduced, and as a result, the head drive transistor 33 is rendered off.
However, in the related head drive circuit, when the head drive transistor is turned off, the power supplied by the head drive power source is not effectively employed. This problem will be described while referring to FIGS. 10A to 10D. In these drawings are presented a diagram showing a simplified head drive circuit, and other diagrams showing the flow of the drive current, as well as its current waveform and the operation of the Zener diode.
First, as is shown in FIG. 10A, when the transistor is rendered on, a drive current i is supplied by a power source Vp in the direction indicated by the arrow, and a head coil is driven. At this time, the collector-emitter voltage (VCE) of the transistor is substantially zero.
To render off the transistor, when the inductive electromotive force that is generated at the coil at the polarities shown in FIG. 10A exceeds the Zener voltage, the Zener diode is rendered conductive, and a base current flows via the Zener diode to the transistor, as is indicated by a broken line in FIG. 10A. Then, the charge on the transistor falls in the linear operation mode, and the energy accumulated in the coil is discharged through the collector and the emitter of the transistor. When the discharge of the energy has been completed, the Zener diode is again rendered non-conductive and the transistor is rendered off.
FIGS. 10B and 10C are graphs showing the changes produced by this process in the collector current i and the collector-emitter voltage (VCE) of the transistor as time elapses. As a result, as is shown in FIG. 10D, of the power (see FIG. 10B) supplied by the power source, power P (=ixc2x7VCE), which is required to render off the transistor, is consumed for heat generation at the transistor as thermal loss represented by Q in the figure.
As is described above, in the related head drive circuit, the power supplied by the power source to render off the transistor is lost and is not effectively employed. Furthermore, since a great deal of heat is generated by the transistor, a cooling member, such as a heat sink, is also required, and accordingly, the size of the package of a power source is enlarged.
To resolve these shortcomings, it is one objective of the present invention to provide a head drive circuit that not only drives the head efficiently, but also reduces the consumption of power, and to produce a compact power source.
To achieve the above objective, according to the present invention, there is provided a head drive circuit for an impact dot printer which performs printing by driving a print wire, comprising:
a DC power source for supplying a power source voltage;
a head coil;
a switching element which is on/off controlled to apply the power source voltage to the head coil for a predetermined time period;
a voltage regulator for converting an input voltage having a value higher than the power source voltage into an output voltage having a value as substantially same as the power source voltage;
a voltage introducer for inputting an inductive voltage, generated in the head coil when the switching element is turned off, into the voltage regulator as the input voltage; and
a voltage returner for feeding back the output voltage of the voltage regulator to the DC power source.
Namely, the head drive circuit is so configured that the voltage regulator returns to the power source the power that accumulates when the switching element (e.g., a transistor) is rendered off.
With this arrangement, the energy that accumulates in the head coil when the switching element is turned off is returned to the power source by the voltage regulator, and is effectively utilized for driving the head coil.
A DC/DC converter or a voltage dropper may be adopted as the voltage regulator.
Preferably, the voltage introducer includes a first rectifier which is rendered conductive when the inductive voltage is generated in the head coil to unidirectionally supply the inductive voltage into the voltage regulator as the input voltage, and the voltage returner includes a second rectifier for unidirectionally supplying the output voltage from the voltage regulator to the DC power source. For example, diodes may be adopted as the rectifiers.
Since the rectifiers (e.g., diodes) required for the prevention of a crosscurrent are provided, the backflow of power, from the input end of the voltage regulator to the switching element, or the inverted supply of power, from the power source to the output end of the voltage regulator, can be prevented.
Preferably, the head drive circuit further comprises an input voltage adjuster for adjusting the input voltage of the voltage regulator so as to have a predetermined value higher than the power source voltage. Specifically, so long as the input voltage of the voltage regulator is raised to a predetermined voltage that only when the switching element is rendered off is higher than the voltage provided by the power source, the power from the head coil can be led to the voltage regulator and can thereafter be returned to the power source by a high induction voltage that is generated at the head coil.
Preferably, the voltage regulator includes an input condenser for smoothing the input voltage thereof. The voltage adjuster includes a charger for charging the input condenser so as to have the predetermined value of input voltage before and while the printing is performed.
Preferably, the charger always applies the predetermined value of voltage to the input condenser.
Alternatively, the switching element is turned on/off repeatedly at a frequency too high to drive the print wire to apply the inductive voltage to the input condenser repeatedly at least before the printing is performed, thereby the switching element and the head coil serve as the charger.
Alternatively, the charging operation using the switching element and the head coil may be used not only for the initial charging performed before the printing is started, but may also be used, as needed, during the printing operation (e.g., following a line return) to supplement the discharging of the condenser.
Alternatively, the charger includes: a charge coil; a coil switching element which is on/off controlled to apply the power source voltage to the charge coil; and an input voltage holder for inputting an inductive voltage, generated in the charge coil when the coil switching element is turned off, to the input condenser. The coil switching element is turned on/off repeatedly to apply the inductive voltage generated in the charge coil to the input condenser repeatedly at least while the printing is performed, thereby the charged voltage in the input condenser is maintained at the predetermined value.
Alternatively, the input voltage holder may be employed not only for supplementary charging during the printing, but also for the initial charging performed before printing is begun.